This invention relates to internal combustion engines and more specifically to devices and methods for altering the acoustic signature of such engines.
A major concern of military operations in the field is the acoutic detection of their fighting vehicles by enemy forces. Engine exhaust noise is the dominant source of noise under idle conditions. Tread noise dominates when vehicles (e.g. tanks) are in motion. The low frequency exhaust pulses of an idling engine are particularly easy to recognize with a simple spectrum analyzer and a microphone.
The periodic nature of the engine exhaust noise at idle results in an audio frequency spectrum dominated by the firing frequency and its harmonics. Internal combustion engines, particularly Otto and Diesel cycle engines, will have a characteristic periodic exhaust noise which includes the firing frequency and higher order harmonics of the firing frequency dependent upon a number of engine parameters. Design parameters such as number and arrangement of cylinder, cylinder dimensions and displacement, exhaust valve design, muffler dynamics and others influence the characteristic exhaust noise emanating from a vehicle. As is well knwon in the art, for every two revolutions of the crankshaft of a four-cycle Otto or Diesel cycle engine, a firing cycle is completed. Thus, an engine idling at 480 RPM will repeat a particular firing pattern and produce a repeatable "noise signature" four times a second. Accordingly, a two-cycle engine idling at 480 RPM repeats its firing pattern eight times a second.
Some engine designs result in uneven or nonuniform firing patterns as a result of the crank pin locations of the crankshaft. Many engines include an uneven firing sequence due to design limitations relating to the number of cylinders and the angle between banks of cylinders, such as is found in a common 90.degree. V-6 engine, which results in an uneven firing engine. An example of an even firing engine serves to illustrate what is meant by an uneven firing engine. In an even firing eight-cylinder engine, a power stroke occurs for each 90 degrees of rotation of the crankshaft of the engine. This is easily determined by knowing the number of cylinders (eight), the fact that a power stroke occurs once for each cylinder over two revolutions of the crankshaft, and that distributing eight power strokes evenly over two revolutions results in a power stroke every 90 degrees to produce even firing engine operation. Thus, it follows that an uneven firing engine does not produce a power stroke at a fixed crankshaft rotational increment.
A device and method for producing a variable idle speed for an internal combustion engine are shown, for example, in copending application Ser. No. 489,684, by P. Hayes and T. Reinhart filed concurrently herewith, titled "Method and Device for Variable Idle Speed Control of an Internal Combustion Engine", the disclosure of which is hereby incorporated by reference.
A method and device for altering the acoustic signature of an engine is needed to prevent audio frequency spectrum identification of military vehicles by enemy forces.